Acta Geodaetica et Geophysica Hungarica 33. (1998)
1. szám - NNSS - Leitinger R.–Spalla P.–Ciraolo L.: Latitude dependent mean ionospheric height – A new approach to the TEC evaluation from NNSS data
62 R LEITINGER et al. accurate) first order approximation it relates to “slant electron content” I/ (the number of free electrons in a tube along the ray path with a cross section of 1 m2) in the following way: (Ne: electron density; d s: straight line element; integration from the satellite transmitting antenna at S to the receiving antenna at R. The “initial phase value” Ф0 is an additive constant which has to be found by means of “evaluation procedures” (see, e.g., Leitinger et al. 1975, Leitinger and Putz 1978, Leitinger et al. 1984). The projection from slant to vertical gives “vertical electron content” (TEC) and therefore is a crucial process which enhances strongly the usefulness of the observations. The geometry for the projection is shown in Fig. 1: S, R and the center of the Earth, C, are in one plane. In this plane we adopt for coordinates the height h and the angle at C, S. Be ß the zenith angle along the ray path, £ the zenith angle of the ray at R. Prom the triangle RCS we gain ß = ( — S. Furthermore we have d s = — d h/ cos ß. Then we have the relation between slant and vertical electron content, I± as follows: Fig. 1. Ray geometry. R: receiver at ground, S: satellite transmitter, C: center of Earth, zenith angle of ray at ground, /3: zenith angle along ray, S: angular distance SR at C Ф + Ф» 27Г / c RJ Neds = s 2тг/ c I/ R s h. 0 1 COS ß h,J Ned h 0 1 cos ßI±1 cosy;I± Acta Geod. Geoph. Hung. 33, 1998